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2091 ~ UI i3 u: LL UI o U ~ UI '" en :Z o ~ en LL o a: o a: a: UI UI a: < ::;) g z is ::; 2.5 1.0 0.5 o 10 20 30 40 STATION RECORD LENGTH, IN YEARS 50 60 70 100 Station-skew coefficient 80 90 Figure 4. Mean square error of station-skew coefficient as a function of station record length for selected values of station-skew coefficient. between -0.5 and 0.5. Generalized skews from the national map (U.S. Interagency Advisory Committee on Water Data, 1982) also were considered appropriate in the frequency analysis of daily ROM discharges downstream from Pueblo Reservoir. Analysis for Adequacy of Record Length Discharge records may fall within a high or low climatic cycle and, therefore, may not accurately repre- sent the long-term conditions. The errors in frequency analysis due to climatic variations usually are small for long periods of record, but may be extremely large for short periods of record (McCain and Jarrell, 1976, p. 3). An analysis was made of the April and May dis- charge records for six stations on the Arkansas River to evaluate the effects of climatic variations and adequacy of record length. This analysis was made by perform- ing a frequency analysis of the April I through May 31 (April-May) discharge volumes; the discharge volumes were derived by summing the April-May ROM dis- charges each year and converting to acre-feet (by mul- tiplying by 1.9835). The variability of April-May discharge volumes with time for the at-Canon City and the combined near-Pueblo/at-Portland stations is shown in figure 7. The estimated April-May discharge volumes at the om EP (0.01 EP volumes) are shown in figure 8; the frequency curves for each station are shown in figure 9. When the period of record at a station is con- sidered, figure 8 seems to indicate (1) that there is a downstream increasing trend in the 0.01 EP volume for the at-Parkdale, at-Canon City, and near-Pueblo sta- tions; and (2) that the om EP volumes for the at- Portland, above-Pueblo, and near-Avondale stations are much larger than the om EP volume indicated by the trend for the first three stations. The frequency curves for the lalter three stations that have the shorter periods of record (fig. 9) may be affected more by the largest discharge volumes (table 2) than the frequency curves for stations that have longer periods of record (the at-Canon City and near-Pueblo stations) (fig. 9). Because the at-Parkdale station is upstream from low- elevation snowmelt and rainfall during April and May, extremely large discharge volumes may be more infre- quent at this station than they are at the downstream ., 10 Uae of Frequency Analyala and the Extended Streamflow PredlcUon ProcedulB 10 Esllmata Evacuation Dataa for the Joint-Use Pool of Pueblo Reservoir, Colorado